This invention relates to integrated circuits such as field programmable gate arrays (FPGAs) which contain an on-chip volatile program memory which must be loaded from an off-chip nonvolatile memory when power is applied before normal operation of the device can commence.
Field programmable gate arrays (FPGAs) constitute a commercially important class of integrated circuit which are programmed by the user to implement a desired logic function. This user programmability is an important advantage of FPGAs over conventional mask programmed application specific integrated circuits (ASICs) since it reduces risk and time to market.
The function of the FPGA is determined by configuration information stored on the chip. Several technologies have been used to implement the configuration store: most notably static random access memory (SRAM), antifuse and flash erasable programmable read only memory (EPROM). The SRAM programmed FPGAs have dominated in the marketplace since they have consistently offered higher density and operating speed than devices using the other control store technologies. SRAM devices can be implemented on standard complementary metal oxide semiconductor (CMOS) process technology whereas antifuse and Flash EPROM technologies require extra processing steps. SRAM devices are normally built on process technology a generation ahead of that used in the other devices. For example, today the most advanced SRAM programmed FPGAs are available implemented on 0.18 micron technology whereas the most advanced nonvolatile FPGAs are on 0.25 micron technology. The smaller transistors available on the advanced processes provide a speed and density advantage to SRAM programmed FPGAs. Additional details of the operation of FPGAs and their control memory are given in standard textbooks including John V. Oldfield and Richard C. Dorf Field Programmable Gate Arrays, published by Wiley-Interscience in 1995.
Unlike antifuse and FLASH EPROM which maintain their state after power is turned off, SRAM is a volatile memory which loses all information on power off. Therefore, SRAM programmed FPGAs must have a configuration bitstream loaded into them immediately after power is applied: normally this configuration information comes from a serial EPROM. A serial EPROM is a small, nonvolatile memory device which is often placed adjacent to the FPGA on the printed circuit board and which is connected to it by a small number of wires. The programming information may also come from a parallel access EPROM or other type of memory or a microprocessor according to the requirements of the system containing the FPGA.
A shortcoming of FPGAs, especially SRAM programmed FPGAs, is a lack of security of the user's design because the configuration bitstreams may be monitored as they are being input into the FPGA. This security issue is one of the few remaining advantages of FPGAs based on nonvolatile memory over SRAM programmed FPGAs. It is very difficult to “clone” a product containing a mask programmed ASIC or one of the nonvolatile FPGAs. Cloning an ASIC involves determining the patterning information on each mask layer which requires specialist equipment and a significant amount of time. It is also difficult to copy configuration information loaded into the nonvolatile FPGA technologies after their “security fuses” have been blown—thus these devices are attractive to customers who have concerns about their design being pirated or reverse engineered. Vendors of FPGAs which use nonvolatile programming memory often refer to the security advantages of their technology over SRAM programmed parts in their marketing literature. As an example, “Protecting Your Intellectual Property from the Pirates” a presentation at DesignCon 98 by Ken Hodor, Product Marketing Manager at Actel Corporation gives the view of the major vendor of antifuse FPGAs on the relative security of antifuse, FLASH and SRAM based FPGAs.
This security problem of SRAM FPGAs has been well known in the industry for at least 10 years and to date no solution attractive enough to be incorporated in a commercial SRAM FPGA has been found. Some users of SRAM FPGAs have implemented a battery back up system which keeps the FPGA powered on in order to preserve its configuration memory contents even when the system containing the FPGA is powered off. The FPGA bitstream is loaded before the equipment containing it is shipped to the end user preventing unauthorized access to the bitstream information. Present day FPGAs have a relatively high power consumption even when the user logic is not operating: which limits the life span of the battery back up. If power is lost for even a fraction of a second the system the FPGA control memory will no longer be valid and the system will cease to function. This raises concerns about the reliability of a system which uses this technique. Thus, this prior art approach to protecting FPGA bitstreams is only applicable to a small fraction of FPGA applications.
As can be appreciated, there is a need for improved techniques and circuitry for secure configuration of FPGAs.